The drought trend and its relationship with rainfall intensity in the Loess Plateau of China

Drought is one of the major natural disasters in the Loess Plateau of China. We aimed to reveal drought occurrence regularity, relations between different magnitudes of drought and precipitation intensity changes. Based on Chinese climate divisions and digital elevation model graphs of the Loess Plateau region, our study area was divided into five subregions by means of cluster analysis of the region's meteorological stations using a self-organizing map neural network. Utilizing daily precipitation data from 72 stations for the 1961-2012 period, and using the number of consecutive days without precipitation as the drought level evaluation, the frequency and differing magnitudes of drought occurrences in each season were analyzed. In addition, the first precipitation intensity after a drought period was analyzed using a Pearson type III frequency curve. Precipitation frequency changes following drought occurrences with different magnitudes in each subregion were analyzed, and the relationship between rainfall intensity and different drought magnitudes was observed, as was the relationship between rainfall intensity and the number of drought occurrences. The results of our study indicate that, first, changes in drought levels in the region within and between years are not obvious, but that changes are significant between decades. Moreover, drought frequency in subregions I, IV and V of our study was higher than in areas II and III. Second, as the drought level increased, the probability of heavy rainstorms and extraordinary rainstorms in the Loess Plateau region became lower, and the frequency of spatial changes at different precipitation intensities varied. The frequency of different precipitation intensities in subregions I, II, IV and V initially showed a decreasing trend, but, with continued development of drought and when extraordinary drought occurred, moderate and heavy rain became an increasing trend, with the frequency of rainstorms and heavy rainstorms decreasing at first, then becoming the same. The probability of moderate and heavy rain in subregion III showed a decreasing trend; that of rainstorms, heavy rainstorms and extraordinary rainstorms initially decreased then became the same. Third, while the frequency of drought occurrences in the Loess Plateau region for the 1961-2012 period showed an increasing trend overall, the extent of the increase differed among the different subregions.